1. Field of Invention
The field of the present invention relates in general to modems and more particularly digital modems.
2. Description of the Related Art
Digital Subscriber Lines (DSL) technology and improvements thereon including: G.Lite, ADSL, VDSL, HDSL, SDSL, MDSL, RADSL, etc. all of which are broadly identified as XDSL have been developed to increase the effective bandwidth of existing subscriber line connections, without requiring the installation of new fiber optic cable. Subscriber lines were originally constructed to handle voice traffic in the narrow band between 300 Hz to 3000 Hz at bandwidths equivalent to several kilo baud. With XDSL significant increases in bandwidth have been made possible by utilizing frequencies higher than the voice band to deliver services such as: data, video, audio etc. Thus an XDSL modem may operate simultaneously with a voice band modem or a telephone conversation.
Within each standard a modulation protocol, e.g. discrete multi-tone (DMT), is specified for establishing a communication channel. DMT modulation involves establishing a communication channel with a plurality of sub-channels each with a center frequency a.k.a. carrier tone. The sub-channels are frequency division multiplexed across the available bandwidth. Each sub-channel may use quadrature phase amplitude modulation (QPAM) to modulate information. The center frequency, a.k.a. tone of each sub-channel serves as the carrier on which QPAM modulation of information is effected. The information modulated on a tone is identified in the frequency domain as a sub-symbol which defines a unique phase and amplitude relationship between the carrier tone and the information modulated on it. Each sub-symbol may be expressed as a complex number. Specific bits of information are converted to a corresponding complex number using a mapping table, which defines for all possible phase and amplitudes supported by the DMT protocol corresponding binary bits. Collectively all the sub-symbols modulated on each tone across a tone set are defined as a symbol, with the symbol rate defined by the corresponding X-DSL standard.
The primary factor limiting the bandwidth or channel capacity of any of the above discussed X-DSL protocols is noise, whether that noise be from echo, channel cross talk, impulse or background sources. Efforts are made throughout the DSL architecture to minimize noise.
The topology of subscriber lines themselves may be used to minimize cross-talk between subscriber lines. Typically, telephone subscriber loops are organized in a binder with 10, 25, or 50 pairs each sharing a common physical or electrical shield in a cable. Due to capacitance and inductive coupling there's cross-talk between each twisted pair even though the pairs are well insulated for DC. The effective cross-talk is reduced, but not eliminated, by adapting different twist distances among different pairs in the binder group. Binder groups are also twisted such that no two groups are adjacent for long runs.
The hybrid circuit which couples the modem to the subscriber line is also designed with noise reduction in mind. The hybrid is basically a bridge circuit which allows bi-directional communication on the subscriber line. When the bridge is balanced the spillover of noise from the modem's transmitted signal to the received signal is reduced. Balancing however requires an impedance match with the telephone subscriber loop which is never fully satisfied because the input impedance of the telephone loop varies from one loop to the next due to differences in length, topology, e.g. bridge-taps, and also temperature variations in the individual subscriber lines.
Typically within each binder of subscriber lines most lines will be in-service' i.e. carrying data for existing users. The remaining lines terminating at a home or business are therefore available to new or existing customers. When a customer orders a new line the Telco provider may engage in a process identified as pre-qualifying or characterizing a line, in order to correctly identify for the new user what level of throughput can be provided on the available subscriber line. Line length and available spectrum may dictate one or another level of service. If, for example high throughput service is requested by the new user, then VDSL provisioning may be called for. The ability of the Telco to provision this line with VDSL will be constrained by line characteristics. The length and quality of the subscriber line will determine whether all or a portion of the two or more upstream and two or more downstream communications VDSL bands will be available. The sheer length of the line results in significant attenuation of the upper communication bands rendering their use impractical. Even on shorter loops the presence of various topological features on the subscriber line such as: bridge-taps, jumpers and changes in wire gauge can also constrain bandwidth. In each instance it is critical in setting up a new customer that the line is properly characterized, to ensure customer satisfaction.
Typically, a process identified as Single Ended Line Testing (SELT) is used by the Telco to characterize the prospective available subscriber line before the customer has even purchased a modem. SELT has proven to be an effective method for characterizing a line whether or not it is terminated at the remote end. SELT subjects the line to passive and active analysis. Passive analysis involves performing a Quiet Line Noise (QLN) measurement on the line under test. QLN measurement takes place without any transmission onto the line under test, e.g. no probe signal. The subscriber line under test is categorized across the entire broadband spectrum for noise level and periodicity of impacting signals. Active analysis involves probing the line with a broadband probing signal and analyzing the resultant reflections, a.k.a. echoes, to further characterize the line. The resultant reflections are received and analyzed to determine line topology including length, bridge-taps, jumpers and changes in wire gauge and their resultant effect on available bandwidth and anticipated throughput level.
What is needed is a SELT process with improved capabilities for subscriber line characterization.